Can I change a post to a page by changing it’s type?

Changing the post_type property of a post is easy, but there are a few con’s you would need to consider here before doing so

• Non hierarchical post types (like post) behaves way different that hierarchical post types (like page). I’m not going to explain everything here, but in general, most importantly, non hierarchical post types have taxonomies bound to it, while hierarchical post types have parent-child relationships. There are still a lot of diffences like templates, post type supports, etc

  Changing a post to a page stuffs this whole relationship and setup

• Even when just changing any non hierarchical post type to another non hierarchical (or hierarchical to another hierarchical post type), you can run into issues. Again look at bullet point one.

In short, you should really avoid blindly changing the post type of any post (specially from hierarchical to non hierarchical and vice versa). You need to make very sure of your structures and how it would be affected should you change the post type of a post.